In "Star Trek: Enterprise", in the final season, they were using remote controlled ships that could generate the illusion that they were Tellarite, Andorian, etc., ships. A logical precursor to the cloaking device of TOS and later periods.

Also, they had secretly infiltrated the Vulcan High Command and were subverting the Vulcan government. AFAIK, that remained secret until "Balance of Terror".

Guy